A scanner system makes use of focusing a reflecting light beam through a photodetector to generate an image signal for further image prosessing. A conventional scanner system includes a light source, a mirror, and a lens set, which are used to guide the reflecting light to a charge couple device (CCD). The charge couple device is utilized to generate an image signal. A pre-processing element is used to respond an image signal and adjust dc gain of the image signal. An analogue to digital converter is used to convert adjusted image signal to a digital signal and a post-processing element to generat an image code by processing said digital signal through highlight, shadow, and Gamma correction.
Typically, the light source is reflected from the surface of a document, then it is reflected against from the mirror and focused by a lens set. The mirror and the lens set are used to guide the light beam to a CCD. Subsequently, the light beam is converted to an image signal by the CCD, and direct current (d.c.) gain of the image signal is adjusted by a pre-processing element, i.e. a d.c. gain voltage amplifier. Then the adjusted image is fed to an analogue to digital converter (ADC) for converting adjusted image signal to a digital signal. The digital signal is fed to a post-processing element to generat an image code by processing the digital signal through highlight, shadow, and Gamma correction.
Generally speaking, the carriage of a scanner is made of aluminum or plastic. Turning to FIG. 1A, it shows a conventional lens holder 10 for holding a lens set 104 and adjusting the focus of the lens 104. A trapezoid shape housing 101 is set on a bottom plate 102. A cylindrical lens fixing element 103 for holding the lens set 104 is conneted to the housing 101. An elastic slice 105 is attached on the outer portion of the lens fixing element 103 to fix the lens set 104 in the lens fixing element 103. In order to focalize the light on the CCD, the lens set 104 can be moved along the axis of the lens fixing element 103.
As shown in FIG. 1B, 1C, the internal radius of the lens fixing element 103 is longer than the external radius of the lens set 104. Therefore, the axis of the lens set 104 can not be aligned with the axis of the cylindrical lens fixing element 103. That is to say the axis of the lens set 104 is not orthogonal to the CCD. Further, the lens set 104 is often shift due to an external force from the elastic slice 105 acts on one end of the lens set 104. Referring to FIG. 1C, another type of the lens holder 10 has a set screw 106 set on the lens fixing element 103 for fixing the lens set 104. It is also the reason to cause the shift of the axis of the lens set 104.
FIG. 2A shows the conditions that the axis of the lens set 104 is not orthogonal to the CCD. FIG. 2B to FIG. 2E show the testing wave shape of a scanner that are analyzed by using an oscilloscope. FIG. 2B is the diagram corresponding to the axis of the lens set 104 that is orthogonal to the CCD. FIG. 2C to FIG. 2E are the diagrams corresponding to the axis of the lens set 104 that are not orthogonal to the CCD. It can be seen in the FIG. 2C to FIG. 2E, the light can not be focalized on the CCD due to the axis of the lens set 104 is shift.